Determining the safe working stress of metals at elevated temperatures



NOV. 22, w, B RR E AL 1,888,755

DETERMINING THE SAFE WORKING STRESS 0F METALS AT ELEVATED TEMPERATURES Filed Oct. 5. 1951 2 Sheets-Sheet 1 W 2 E In/moms 1f g, 7" vi Arron/ray.

Nov. 22, 1932. w, BARR ET AL 1,888,755

DETERMINING THE SAFE WORKING STRESS OF METALS AT ELEVATED TEMPERATURES Filed Oct. 3, 1951 2 Sheets-Sheet 2 Fig.2.

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T2 5 Mg m ve/rrm Avromrzv Patented Nov. 22, 1932 t was UNITED STAT S PATENT. OFFICE WILLIAM am, or min-on, am) WILLIAM nnwmn naanan'r'r, or emsoow, scormn. assmnons 'ro COLVILLIES Lnu'rnn, or GLASGOW, SCOTLAND na'rnamme rim sun woaxme amass or METALS AT nnnvarnn rmnna'ruans Application filed October 8, 1931. Serial No. 566,754.

This invention relates to an improved means of and apparatus for determining the safe working stress of metals such as iron and steel and also non-ferrous metals and their alloys which are subjected to stress at elevated temperatures; it being highly desirable to know the maximum stress which a metal can withstand indefinitely at a given temperature without undergoing any pronounced creep, or plastic extension, due to metals at elevated temperature can be the combined effects of stress and temperature.

An object of the invention is to make provision whereby the safe working stress dof etermined with suflicient accuracy to meet all practical requirements and also within the space of a comparatively short time.

By careful experiments, we have ascertained that, under the continued action of gives a decreased rate of creep; or, in other words, the lower the initial stress the less will be the creep at the end of a given time. Thus, it follows that there will be a certain initial stress which will give no measurable creep during the said period and which therefore will be a safe working stress.

3 creeps,

Applying the knowledge derived from our experiments to the attainment of the objects of the present invention, specimens of a metal to be tested are heated to a selected temperature and subjected to different initial stresses, and, for a selected period, the temperature is maintained constant while the stresses gradually decrease as the metal elongates or the rates of decrease being proportionate to the creep experienced by the re spective specimens. At the end of the said period, the decreases in stress experiencd by the respective specimens are evaluated, and the safe working stress is determined mathematically, preferably graphically, from the relationships between the initial stresses and the corresponding decreases in stress.

In order to control the decrease in stress, and at the same time to measure the stress, we preferably make use of weigh bars to which we operatively connect the res ective specimens. By the term weigh ar is meant a calibrated elastic body the physical properties of which are known and which accordlngly serves as a measure of the stress to which the specimen operatively connected to it is being subjected.

In order that the method and apparatus whereby our objects are attained can be clearly understood, reference will now be made by way of example to the accompanying drawin s, in which 1g. 1 is a sectional elevation showin diagrammatically apparatus for use in etermining the safe working stress of metals ace cording to this invention.

Fig. 2 is a table of values obtained in a series of tests performed at different temperatures by the use of the said apparatus.

Fi 3 is a view showing how the safe working stresses at the respective temperatures are graphically determined from the said values.

Referring first to Fig. 1, the apparatus therein shown comprises a battery of three units, and since these units are identical only one of them will be particularly described.

As shown, each unit includes a rigid rectangular frame 1 formed at the top and bottom with openings 1 and 1". A bar 2 passing through the top opening of the frame 1 has a square-section portion 2a which fits neatly through a square hole in a ring plate 3 secured to the top of the frame. The bar 2 is thus permitted to slide endwise through the plate but is constrained against turning movement. The upper part of the bar 2 is screw-threaded, being in engagement with a nut 4 on top of the plate 3, and a thrustbearing at, for example a ball-bearing, may be interposed between the plate 3 and nut 4. A bar 22 is suspended from below the bar 2 by a universal joint 23. A round bar 5 is adapted to be passed through the lower openin of the frame 1, to the bottom of which a plate on the bottom end of the bar 5. There is also provided an intermediate connectin bar 7.

The unit includes an electric urnace 8 i which can be set up within the frame 1 upon a platform 9 supported by flexible suspension cables 10 passing over the, top of the frame.

The letters S and W respectively indlcate the s ecimen to be tested and a calibrated steel ar constituting a weigh bar. i

There is also provided a delicate extensometer of any a ropriate form,'for exam le as indicated gy 11.

efo're describing a test as performed by use of the battery of units, we shall describe what takes place in a single unit preparatory to and during the test.

The parts 2, 5, 7, S and W are assembled as shown in the drawings, the specimen S being secured to the bars 22 and 7, the weigh bar W bein connected between the bars 5 and 7, and t e extensometer being fitted to the weigh bar. In order that the parts W and S can be firmly secured, they may for example be rovided with screw-threaded extensions (w ose outlines are shown in dotted lines) adapted to be screwed into screw-threaded holles7 in the adjoining ends of the parts 2, 5 an The electric furnace 8 is energized and the current regulated to, and kept at, that value which maintains the specimen S at a selected temperature, the current being thermostatically controlled in known manner.

The nut 4 is turned so as to place the specimen and weigh bar under a redetermined initial stress, the magnitude 0 which would be greater than the roughly anticipated magnitude of the safe working stress at a selected temperature. The magnitude of the ini tial stress imposed by turning the nut 4 is directly obtainable from the reading indicated by the extcnsometer 11, as said magnitude isa known function of the elastic strain or extension imparted to the weigh bar W.

As the test proceeds the specimen S gradually elongates, or creeps, due to the combined influences of stress and temperature. As the cimen creeps, the stress to which the w'eig bar W is subjected becomes proportionately decreased, and consequently the specimen S is automatically subjected to a gradually decreasing stress, the magnitude of the decrease being continuously obtainable from theextensometer reading.

In carrying out the actual test, three similar specimens of a metal whose safe working stress is required are taken and are fitted into the respective units. The temperature of the three electric furnaces 8 is so regulated that the three specimens are all maintained at precisely the same predetermined temperature throughout the test. At the commencement of the test the three specimens are subjected to materlally different initial stresses whose values are noted. The test may endure for a period of time which may beabout two days. We recommend this period, firstly, because of the need for a comparatively short test and, secondly, because a large proportion of the creep takes place in the first forty eight hours.

At the end of the test period, the extensometers are read and the values of the respective decreases in stress are noted. These values, together with those of the corresponding initial stresses, sulfice to enable one to determine the safe working stress of the metal under test, at the respective temperatures, as will now be explained.

As previously stated, the lower the initial stress, the less will be the rate of creep, and accordingly the decrease of stress at the end of the test will also be less. We have ascertained that the mathematical law governing the relationship between the initial stresses and the corresponding decreases in stress can be graphically represented by a straight line. Thus it will be clear that two or more values of both initial stress and decrease in stress over a given period enable one to calculate, or estimate graphically, that initial stress which will give no measurable decrease in stress, the said initial stress being as aforesaid the safe working stress of the metal under test.

' In order that the above statements can be clearly understood, we shall describe an actual series of tests with reference to Figs. 2 and 3.

-The metal under test was mild steel containing 11% carbon, .01% silicon, 018% sulphur, .008% phosphorus and .56% manganese. A series of tests as above described was carried out at temperatures of 400 (1., 450 C. and 500 C. and one further test was carried out with the use of only two units of the battery at 550 C. The values in tons per square inch of the initial stresses and decreases in stress are tabulated in Fig. 2. Graphs for the series of tests are shown in Fig. 3, the values of the decreases in stress being the ordinates and the values of the initial stresses being the abscissae. It will be clear that the point where each of the respective graphs meets the base line denotes the maximum initial stress which will give no measurable decrease in stress at the temperature concerned. and also that this maximum initial stress will be the safe working stress.

It will be seen from Fig. 3 that the values in tons per square inch of the safe working stress for the series are roughly 3.75, 2.3, 1 and .5, these values being arranged in the same order as the tests tabulated in Fig. 2.

It will be clear that a safe working stress can be obtained merely by performing tests on two specimens. It will however be apparcut that more accurate results can generally be obtained by having at least three units in the battery. It will also be apparent that tests at one temperature need not be performed concurrently so long as the period of duration is the same, but wnere speed is an important factor all tests at one temperature should be carried out concurrently by use of a battery of units.

o claim v 1. A method of determining the safe working stress of metals at a selected temperature, which consists in subjecting specimens of a metal while at said temperature to different initial stresses, allowing said stresses to decrease under the creep of the specimens themselves for a selected period of time, ascertaining the different creep values at the end of said period, and evaluating the safe working stress of the metal at said temperature from the relationship between said values and the corresponding initial stresses.

2. A method of determining the safe working stress of metals at a selected temperature, which consists in taking separate specimens of a metal, heating these specimens to and maintaining them at said temperature, elastically subjecting the respective specimens to ditierent initial stresses whereby they experience creep, allowing said stresses gradually to decrease under the automatic action of said creep for a selected period of time, ascertaining the respective decreases in stress at the end of said period, and evaluating from said decreases and the corresponding initial stresses the safe working stress of the metal at said temperature.

3. A method of determining the safe working stress of metals at a selected temperature, which consists in subjecting specimens of a metal while at said temperature to different initial stresses, allowing sai stresses to decrease under the creep of the specimens for a selected period of time, ascertaining the diflerent creep values at the end of said period, ascertaining the decreases in stress corresponding respectively to said creep values, and graphically determining the safe working stress of the metal at said temperature from the relationship between said decreases and the corresponding initial stresses.

4. A method of determining the safe working stress of metals at a selected temperature, which consists in taking separate specimens of a metal, heating these specimens to and maintaining them at said temperature, elastically subiecting the respective specimens to ditferent initial stresses whereby they experience creep, allowing said stresses gradually to decrease under the automatic action of said creep for a selected the decreases in stress corresponding respectively to said creep values, and graphically determining from said decreases and the cor responding initial stresses the safe working stress of the metal at said temperature.

A method of determining the safe working stress of metals at a selected temperature. which consists in concurrently subjecting specimens of a metal while at said temperature to difierent initial stresses, all said stresses to decrease concurrently under the creep of the specimens for a selected period of time, ascertaining the difierent creep values at the end of said period, and evaluating the safe working stress of the metal at said temperature from the relationship between said values and the corresponding initial stresses.

(3. A method of determining the safe working of metals at a selected temperature, which consists in taking separate specimens of a metal, concurrently heating these specimens to and maintaining them at said temperature, elastically subjecting the respective specimens to different initial stresses whereby they concurrently experience creep, allowing said stresses gradually to decrease under the automatic action of said creep for a selected period of time, ascertaining the respective decreases in stress at the end of said period. and evaluating from said decreases and the corresponding initial stresses the safe working stress of the metal at said temperature.

In testimony whereof we afiix our signatures.

WILLIAM BARR.

\VILLIAM EDWARD BARDGETT.

period of time, noting the difierent creep values at the end of said period, ascertaining allowing 4 

